Effects of iontophoresed opioids on physiologically characterized laminae I and II dorsal horn neurons in the cat spinal cord.
The objective of the present study was to determine in vivo the effects of opioids applied locally via microiontophoresis on physiologically characterized laminae I and II dorsal horn neurons in the cat spinal cord. Experiments were performed on pentobarbital-anesthetized or decerebrate, spinalized cats. The effects of morphine (MOR), [D-Ala2,methyl-Phe4, Gly-ol5]enkephalin (DAGO) and naloxone (NALO) on spontaneous- and D,L-homocysteic acid-evoked unit activity were examined for 94 laminae I and II dorsal horn neurons. MOR, DAGO and NALO produced mixed effects (i.e., excitation or inhibition) on unit activity; however, the majority of cells examined (67%) were inhibited. Whether MOR, DAGO or NALO exerted excitatory or inhibitory influences on unit activity did not depend on the modality of the neuron (66% of high threshold units, 56% of multireceptive units and 69% of low threshold units were inhibited; some cells of each modality also were excited). NALO also was found rarely to be neutral; 75% of the cells examined were inhibited and 16% excited by NALO. Since NALO was found to itself have significant effects on laminae I and II dorsal horn unit activity, the ability to use NALO as an opioid antagonist to MOR and DAGO was confounded.
Jones
S L
SL
Department of Pharmacology, College of Medicine, University of Oklahoma, Oklahoma City 73190.
Sedivec
M J
MJ
Light
A R
AR
D000818:Animals; D002415:Cats; D003655:Decerebrate State; D004558:Electric Stimulation; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007478:Iontophoresis; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects*; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects*
D004745:Enkephalins; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009270:Naloxone; D009020:Morphine